Earthquake Of April Research Articles

Determination of Epicenter and Comparison of some Empirical Formulae with Observed Data of Acceleration from Two Events in Himalaya

Abstract Two earthquakes of April 26 and September 10 were recorded by strong motion array's in India during the year 1986 by Kangra and ShilIoog array respectively. Mean peak horizontal accelerations are in the range of 157 to 2217 mm/sec in case of the April event, whereas it is 163 to 1234 rnmlsec2 for the September one. Epicentral information (DEQ) has been derived using P and S wave arrival times of strong motion accelerograph stations and compared with that reported by IMD and USGS. Epicentral distance and focal depth are better indicated by the strong motion array data. Focal parameter obtained for the earthquakes differ from that reported by USGS. For the April earthquake, USGS epicenter (E2) lies around 10 km southeast of DEQ epicenter (E3). Focal depth obtained in this study is only around 7 km against 33 km given by USGS. But in the case of the September event epicenter location by DEQ (E3) and USGS (E2) lie close to each other but focal depths were different around 28 km (DEQ) against 43 km (USGS). Error parameters, like RMS, ERH, ERZ are very low in both the cases. McGuire's and other empirical formulae were used to estimate acceleration at a particuIar site. For the April event McGuire's estimate is very close to tbe observed acceleration when DEQ hypocenter was taken into consideration. However, accelerations estimated for the September event do not conform to the observed ones, eventhough some values are quite cIose.

Reservoir-associated earthquakes of April 1983 in Western Thailand: source modeling and implications for induced seismicity : Chung, W Y; Liu, C Pure appl geophys V138, N1, 1992, P17–41

Reservoir-associated earthquakes of April 1983 in Western Thailand: source modeling and implications for induced seismicity : Chung, W Y; Liu, C Pure appl geophys V138, N1, 1992, P17–41

Aftershocks of the San Marcos earthquake of April 25, 1989 (M S =6.9) and some implications for the Acapulco-San Marcos, Mexico, seismic potential

Aftershock activity following the April 25, 1989 (M S = 6.9) earthquake near San Marcos, Guerrero, Mexico, was monitored by a temporary network installed twelve hours after the mainshock and remaining in operation for one week. Of the 350 events recorded by this temporary array, 103 were selected for further analysis in order to determine spatial characteristics of the aftershock activity. An aftershock area of approximately 780 km2 is delimited by the best quality locations. The area of highest aftershock density lies inside an area delimited by the aftershocks of the latest large event in the region in 1957 (M S = 7.5) and it partially overlaps the zone of maximum intensity of the earlier 1907 (M S = 7.7) shock. Aftershocks also appear to cluster close to the mainshock hypocenter. This clustering agrees with the zone of maximum slip during the mainshock, as previously determined from strong motion records. A low angle Benioff zone is defined by the aftershock hypocenters with a slight tendency for the slab to follow a subhorizontal trajectory after a 110 km distance from the trench axis, a feature which has been observed in the neighboring Guerrero Gap. A composite focal mechanism for events close to the mainshock which also coincides with the zone of largest aftershock density, indicates a thrust fault similar to the mainshock fault plane solution.

The Cape Mendocino, California, Earthquakes of April 1992: Subduction at the Triple Junction

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America-Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

Structural damage aspects of the April 22, 1991 Costa Rica earthquake

The magnitude M = 7.4 earthquake of April 22, 1991 occurred in a comparatively lightly inhabited region of Costa Rica. Despite dramatic geomorphological effects, including extensive landslides in the mountainous epicentral region, and uplift of the East Coastline by up to 1.5m, structural damage was comparatively light. A number of warehouses, and reinforced concrete buildings in or near the Port of Lim6n collapsed or were badly damaged, and a considerable number of houses supported on tall wooden piles failed. Of greatest interest was the failure of a number of modem highway bridges, due primarily to liquefaction of foundations, or excessive movement of soft soil layers in embankments.

The reservoir-associated earthquakes of April 1983 in western Thailand: Source modeling and implications for induced seismicity

On 22 April 1983, a very large area of Thailand and part of Burma were strongly shaken by a rare earthquake (mb=5.8,Ms=5.9). The epicenter was located at the Srinagarind reservoir about 190 km northwest of Bangkok, a relatively stable continental region that experienced little previous seismicity. The main shock was preceded by some foreshocks and followed by numerous aftershocks. The largest foreshock ofmb=5.2 occurred 1 week before the main shock, and the largest aftershock ofmb=5.3 took place about 3 hours after the main shock. Focal mechanisms of the three largest events in this earthquake sequence have been studied by other seismologists using firts-motion data. However, the solutions for the main shock and the largest aftershock showed significant inconsistency with known surface geology and regional tectonics. We reexamined the mechanisms of these three events by using teleseismicP-andS-waveforms and through careful readings ofP-wave first motions. The directions of theP axes in our study range from NNW-SSE to NNE-SSW, and nodal planes strike in the NW-SE to about E-W in agreement with regional tectonics and surface geology. The main shock mechanism strikes 255°, dips 48°, and slips 63.5°. The fault motions during the main shock and the foreshock are mainly thrust, while the largest aftershock has a large strike-slip component. The seismic moment and the stress drop of the mainshock are determined to be 3.86×1024 dyne-cm and 180 bars, respectively. The occurrence of these thrust events appears to correlate with the unloading of the Srinagarind reservoir. The focal depths of the largest foreshock, the main shock, and the largest aftershock are determined to be 5.4 km, 8 km, and 22.7 km, respectively, from waveform modeling and relative location showing a downward migration of hypocenters of the three largest events during the earthquake sequence. Other characteristics of this reservoir-induced earthquake sequence are also discussed.

The large normal‐faulting Mariana Earthquake of April 5, 1990 in uncoupled subduction zone

A large, Ms = 7.5, shallow earthquake occurred beneath the Mariana trench on April 5, 1990. From the relocated aftershock distribution, the fault area is estimated to be 70 × 40 km2. A tsunami observed on the Japanese islands verifies that the depth of the main shock is shallow. For waveform analysis, we use long‐period surface waves and body waves recorded at global networks of GDSN, IRIS, GEOSCOPE and ERIOS. The centroid moment tensor (CMT) solution from surface waves indicates normal faulting on a fault whose strike is parallel to the local axis of the Mariana trench, with the tension axis perpendicular to it. The seismic moment is 1.4 × 1020 Nm (× 1027 dyn.cm) which gives Mw = 7.3. Far‐field P and SH waves from 13 stations are used to determine the source time function. Since the sea around the epicentral region is about 5 km deep, body waveforms are contaminated with water reverberations. The inversion results in a source time function with a predominantly single event with a duration of 10 sec, a seismic moment of 2.1 × 1020 Nm, and a focal mechanism given by strike = 198°, dip = 48°, slip = 90°. The short duration indicates a small area of the rupture. The location of the main shock with respect to the aftershock area suggests that the nodal plane dipping to the west is preferred for the fault plane. The local stress drop of the single subevent is estimated to be 150 MPa (1.5 Kbars). The Mariana earthquake is considered to have occurred in an uncoupled region, in response to the gravitational pull caused by the downgoing Pacific plate.

Tsunami from the Mariana Earthquake of April 5, 1990: Its abnormal propagation and implications for tsunami potential from outer‐rise earthquakes

The tsunami generated by the Mariana earthquake of April 5, 1990 was observed on the Japanese and Pacific islands as far as Hawaii. The observed tsunami amplitudes are not a simple function of distance from the source but vary with large‐scale bathymetry. Numerical computations of tsunami propagation are made for actual bathymetry and the computed amplitudes are compared with the observations. From the comparisons, the seismic moment is estimated to be 1.4 × 1020Nm, very similar to that from seismic waves, and indicates that the seismic waves and tsunami are equally excited. The numerical computations also show that the tsunami propagated toward the Japanese coast through two different paths: one is through the trench system with high velocity and small amplitude, the other along the ridge system with low velocity but large amplitude. The two kinds of tsunami are identified on recently‐installed ocean bottom pressure gauges. Since the Mariana earthquake was an outer‐rise earthquake in a weakly coupled subduction zone, where the size of outer‐rise event is larger than underthrusting events, it is possible that even larger earthquakes occur in this region. The tsunami potential from such events must be considered, including the unusual tsunami propagation.

Seismology and Tectonics

The earthquake of April 22, 1991 was located in the eastern foothills of the Cordillera de Talamanca in Valle de la Estrella with epicentral coordinates of 9°38.62′N and 84°5.92″W and a focal depth of 21.5 km (Figure 2-1). An analysis of the seismograms recorded at teleseismic distances indicates that the causative fault was a shallow-angle thrust fault striking in a southeast-northwest direction and dipping at less that 30° gently towards the southwest. The lower block moved in a westerly direction while uplifting the upper-block east coast of Costa Rica by 1 to 1.5 meters. The plane of this fault rupture may either have reached the surface some distance offshore, or, possibly, failed to reach the surface. Aftershock data are consistent with this interpretation.

The Srinakarin Reservoir earthquake, Thailand

The Srinakarin Reservoir is situated in the west of Thailand, about 190 km northwest of Bangkok. It was completed in 1981. On 22 April 1983, an earthquake of M b 5.8 occurred at the border of the end of the a reservoir (approximately 50 km away from the dam). From comparisons of the typical characteristics of reservoir earthquakes and values of probabilistic prediction, two-step comprehensive fuzzy evaluation and estimation of maximum magnitude, we infer that this earthquake of April 1983 has been reservoir-induced. If this is true then this earthquake represents one of the most important reservoir earthquakes in the world in the 1980s.

Evidence for the variation of stress drop between normal and thrust faulting earthquakes in Italy

The horizontal components of 45 strong motion accelerograms recorded in Friuli, Italy, from May to September 1976, have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes. Two different stress parameters, the Brune stress drop Δσ and the apparent stress σa, have been considered. The Friuli main shock has a strong stress drop (Δσ ≈ 800 bars), while the aftershocks have stress drops which range from 250 to 400 bars. Five accelerograms written by the Montenegro, Yugoslavia, earthquake of April 1979 have also been analyzed. This thrust faulting earthquake had a stress drop of Δσ ≈ 900 bars. The stress drops of these thrust faulting earthquakes are compared to the stress drops determined by Rovelli et al. (1988a) for nine normal faulting earthquakes which occurred in the Apennines region from 1979 to 1984. The radiated energies and apparent stresses have also been calculated for the set of Apennine earthquakes. The thrust faulting earthquakes in the Alpine‐Dynaride region exhibit higher values for both the Brune and the apparent stress than the normal faulting earthquakes in the Apennines region. Excluding the Friuli and Montenegro main shocks, the Brune stress drops of the thrust faults are approximately 3 times those of the normal faults, while the apparent stresses are twice as large. This difference is statistically significant at a confidence level of 95%. The same difference also emerges from the scaling of the peak ground motions recorded in these two regions, suggesting an enhanced severity of the seismic input in the Alpine‐Dynaride environment compared with the Apennines.

Borehole tensor strain measurements in California

Two continuous borehole plane strain monitoring sites have been operational in California since late 1983, using borehole tensor strain monitors implanted at a depth of 150 m. Shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals. At Pinon Flat Observatory, data indicate a constant shear strain accumulation of 0.6 microstrain per annum with the axis of maximum compression oriented 50° ± 5° west of north. This result differs significantly from regional geodetic estimates, the amplitude being dominated by continued viscoelastic response of the hole. Variations of measured strain rate exceeding 100 nstrain/yr would be significant at this instrument site. At San Juan Bautista, measurements of the angle of maximum compression (10° ± 3° west of north) agree closely with previous geodetic and hydrofracture estimates. After 2 years of operation the residual shear strain rate is 1 μstrain/yr. Preliminary analysis of strain steps observed at San Juan Bautista during the Morgan Hill earthquake of April 24, 1984, show good agreement with calculations from seismically determined source parameters for this event.

The earthquake of April 30, 1954, in Sophades (Central Greece)

Summary. The Sophades earthquake of 1954 April 30 marked the beginning of a series of earthquakes in Central Greece that culminated in the destructive events of 1955 and 1957 around the town of Volos. A thorough documentation of the surface expressions of the Sophades earthquake was recently discovered in the diaries and photographic archives of Yanni Papastamatiou* . The surface manifestations included fractures and other ground disturbances. This evidence is summarised in this report and is combined with geological and geophysical information available in the area. The combined evidence suggests two possible fault zones, that could be responsible for the earthquake. Both zones are consistent with a northerly extensional deformation, similar to the one suggested for the region affected by the 1978 Chalkidiki earthquakes in N. Greece.

Effects of the 1906 Earthquake on the Bald Hill Outlet System, San Mateo County, California

The San Andreas dam and associated water distribution system were built in 1868–70 to collect, store, and distribute domestic water for use in San Francisco. Although the dam and reservoir are situated in the valley of the same name in close proximity to the San Andreas fault, they were constructed before the San Andreas fault was recognized. Following the earthquake of April 18, 1906, it was discovered that a brick forebay and other parts of the reservoir outlet system were in the slip zone of the San Andreas fault. The original outlet through which water was directed to San Francisco consisted of two tunnels joined at the brick forebay; one tunnel extends 2,820 ft to the east under Bald Hill on Buri Buri Ridge, and the other tunnel intersects the lake bottom about 250 ft west of the forebay. In 1897 a second intake was added to the system, also joining the original forebay. No major damage to this outlet system was reported in 1906, even though offsets of 7 to 10 ft were reported short distances to the southeast and northwest along the fault. During the present study the accessible parts of this original outlet system were examined with the hope of learning how the system had been affected by fault slip in 1906. The west end of the accessible tunnels was found to have been displaced at least 8.3 ft right laterally, along with a small component of east-side-down vertical movement. The tunnels are deformed over a width of at least 75 ft, and it is possible that the zone of deformation may be more than twice that width. It appears that sections of the rigid tunnels bent and rotated in the zone of deformation rather than shearing along a single fracture.

The Morgan Hill Earthquake of April 24, 1984—Effects on United Technologies Corporation

United Technologies Chemical Systems Division, a manufacturer of rocket engine propellants, was situated in close proximity to the Calaveras fault. Soil conditions and fault rupture mechanisms produced anomalously high ground motion at the site. The main plant complex insisted of more than 130 buildings, constructed within the past 25 years to building code standards that considered earthquakes. Damage to buildings was not extensive and was limited to the structures founded in the valley on loose alluvial deposits. Most of the damage was associated with nonstructural building elements and equipment. Estimated repair costs exceeded $1.25 million. No lives were lost and only minor injuries were experienced. Important observations include: (1) soil amplification effects increased damage, and (2) that persons who had received blast response training responded far better than those who were not exposed to such training.

The Morgan Hill Earthquake of April 24, 1984—Summary of Highway Bridge Damage

Only one bridge of 39 investigated was observed to sustain major structural damage. The county bridge carrying East Dunne Avenue over Anderson Reservoir, Bridge No. 37C-166, was closed to traffic by a rock slide and suffered major damage.

The Morgan Hill Earthquake of April 24, 1984—Investigation of Lifelines

The overall response of all lifelines was good although there was some significant damage. In the power system there was significant damage to 500 kV circuit breakers at two substations, some other minor substation damage, and some damage to feeders in the distribution system. The substation damage did put a segment of one of the AC Pacific Interties out of service for a couple of days but the remaining intertie was able to carry the load. The loss of feeder lines caused local disruption of power for up to 5 hours. In water and sewage systems two 8 inch transit water lines broke in Morgan Hill and a break in an 8 inch pipe to a 350,000 gallon tank caused a large water loss. Five 6 inch water line couplings disengaged. There were numerous water service connection failures in the high damage area. There was no significant damage to communication systems but there was a significant downgrade in the quality of service due to system overload. Calls from outside the area were limited by AT&T. Damage to highway systems was limited to that caused by a rock slide which blocked E. Dunne Avenue near the Anderson Reservoir and damaged a nearby bridge over the reservoir. One bridge span moved one foot near an abutment and caused the adjacent pier to move out of plumb by almost one foot. The road surface on the crest of the Anderson Dam was cracked but there was no damage to the dam. In the natural gas system there was no damage to the utilities part of the system. A large fire in San Jose was caused by a gas leak in piping to a gas fired industrial heater. There was an explosion and fire in a mobile home and numerous leaks associated with toppled water heaters. There were numerous leaks beyond service connections to homes in the high damage area.

The Morgan Hill Earthquake of April 24, 1984—Effects on Facilities of the Santa Clara Valley Water District

The Santa Clara Valley Water District (the District) is a public agency with flood control and water supply management authority in Santa Clara County, California. The District operates ten dams and reservoirs, two water treatment plants which supply potable water, 74 miles of large-diameter water distribution pipelines and 34 miles of small canals. Flood control facilities include 45 miles of streams channeled by levees and 17 miles of bayfront levees. Immediately after the earthquake, the District's Emergency Operations Center was activated. Major dams were inspected immediately by operations staff in accordance with standing instructions. The fundamental operations of the District were not interrupted by the earthquake in any significant way. Both water treatment plants operated continuously during and after the earthquake. Damage was limited to minor (functionally insignificant) surficial cracking at two embankment dams. Electronic distance measuring surveys indicated that a large existing landslide, on which one of the water treatment plants is located, moved as much as 13 mm as a result of the earthquake.

The Morgan Hill Earthquake of April 24, 1984—Emergency Preparedness and Response

The Morgan Hill earthquake of April 24, 1984 tested the response structure of Santa Clara County and several cities within the County. While there was no damage to the communications and command infrastructure, the response was seriously disrupted by overloading of the telephone system, and the lack of accurate damage intelligence. Analysis of the response to the earthquake has served to emphasize the need for improved preparedness activities at the local and regional level, the involvement of individuals, businesses, and schools in preparedness planning, and for improvements in communication systems and procedures.

The Morgan Hill Earthquake of April 24, 1984—Effects on Industrial Facilities, Buildings, and Other Facilities

The response of the equipment commonly found in power plants and industrial facilities to the Morgan Hill earthquake of April 24, 1984 is reviewed. Brief descriptions of the general effects of the earthquake are given, including strong ground motion recordings and impacts on commerical and residential structures and their contents. Four sites near San Jose and seven sites near Morgan Hill are examined in detail. These include two wineries, a community college, an industrial park, a software development laboratory, a chemical manufacturing complex, and an electrical substation. Both damaged and undamaged equipment is examined. Generally equipment performed quite well in this moderate earthquake. Seismic damage to well-engineered systems and structures was minimal to non-existent.